An Experiment in Onion Spacing

Updated: Nov 28, 2020

We transplant a lot of crops around here if you haven't noticed yet. The numerous benefits of transplanting include improved consistency, better germination indoors, earlier starts, quicker crop transitions in the field, and exceptionally healthy plants. Since I appreciate all of those benefits, we won't stop transplanting our crops any time soon. However, there are still small parts of the process that are open to improvement. One of those areas is plant spacing.

For the last few growing seasons, we've settled on an onion spacing that we like, which is 4 rows across our 30 inch wide beds with 6 inch spacing between plants in the rows. That gives us about 160 plants in each of our 50 square foot beds, or 3.2 plants/square foot. The experiment I will share with you today is not about changing the onion plant density, but rather about how we can achieve that same density with decreased labour and cost. You see, as long as we keep that average plant density in our onion beds the same, there is some wiggle room as to how we get those onions into the bed.

Enter the multiplant soil block...

A multiplant soil block is just a soil block that contains multiple seedlings. Some crops are content to grow in close proximity with each other, and in these cases we can actually transplant these multi-plant soil blocks directly into field without thinning and let the plants mature in a small cluster. We commonly use multi-plant blocks with beets, spinach, turnips, peas, beans, and onions.

We start and raise these multiplant soil blocks just like a single plant soil block. We just seed each block with more seeds and increase the block size a little to allow more room for the additional plants. The photo below shows three 1 inch soil blocks that each contain one plant next to one 1.5 inch soil block that contains 3 plants to give you an idea of the size difference. The large single block contains roughly the same volume of potting soil as the 3 smaller blocks combined.

Three single plant blocks compared with one multiplant block.

So if we plant a multiplant soil block in place of every single plant soil block, our yield will increase right? Not so fast. There would be a lot more plants in our bed if we did this, but more plants doesn't necessarily mean a higher yield because all of those plants would have to compete with each other for resources. The graph below shows the general idea with plant spacing and its effect on overall yield. If we increase the plant spacing too much, yield can actually begin to decrease and we certainly want to avoid that. Our aim with this onion transplanting experiment is to keep the plant spacing in the sweet spot but use less labour to achieve this spacing by using multiplant soil blocks.

Yield increases with plant density but only to a point. Beyond that yield begins to decrease and the incidence of disease and pest problems increases.

If we transplant our multiplant soil blocks further apart than our single plant soil blocks, we can maintain the same average plant density in our beds with far fewer blocks to transplant. It takes 160 single plant blocks to fill one 50 square foot bed but only 60 multiplant blocks to fill the same space, so there is no question that these multiplant soil blocks can save labour. The question I wanted to investigate was whether or not the use of these multiplant blocks impacts the overall yield. If yield is going to be seriously compromised, then I'd rather just stick with the single plant blocks. Some experimentation was in order.


How will the use of multiplant onion blocks impact the overall yield/bed as compared to single plant blocks?


I have observed that the average onion size decreases a little when onions are grown in clusters so I suspect that an overall decrease in yield will be observed from the multiplant onion beds if I keep the plant density the same.


We used two onion varieties for this experiment: Cabernet and Patterson. For each onion variety, we planted separate beds with single plant blocks and multiplant blocks. The single plant blocks were prepared by seeding single seeds into 1 inch blocks and the multiplant blocks were prepared by seeding 4 seeds into 1.5 inch blocks. These blocks were all seeded indoors on March 16 and eventually hardened off and transplanted into the field. All of the Cabernet onions were transplanted on May 4 and all of the Patterson onions were transplanted on May 12.

Two trays of single plant blocks were needed to fill a bed while only one tray of multiplant blocks was needed for the same space.

The single plant onions were planted in 4 rows 6 inches apart with plants spaced 6 inches apart in row, giving us 160 plants per bed. The multiplant onions were planted in 2 staggered rows 12 inches apart with blocks spaced 8 inches apart in row. Since not all of the seeds germinated perfectly, some multiplant blocks contained 2 or 3 plants. I planted 60 multi-plant blocks per bed estimating that there were an average of 3 plants per block, giving us 180 plants per bed. I know that's not exactly the same density as the single plant beds, but that's as much precision as I had time for this spring, and I knew I could still count the onions harvested at the end of the trial.

Single plant onion bed
Multiplant onion bed

Beds were irrigated with 2 lines of drip tape as shown in the photos above. While the tests of Cabernet and Patterson onions were conducted at different plots, the beds at each plot received the exact amount of irrigation.

Patterson onion beds on June 29.
Single plant Cabernet bed just before harvest on Jul 31.
Multiplant Cabernet onions right before harvest on July 31.
Patterson beds right before harvest on August 19. Multiplant bed is in the middle.

Eventually, it was time to pull the crops out of the field for curing. The Cabernet onions were harvested on July 31 and the Patterson onions were harvested on August 19. Both crops were moved directly to our onion drying rack to cure, delaying counting and measurements until after the curing process. Dry weight of each bed was measured 3 weeks after their harvest date.

Cabernet onions on the drying rack. Mutiplant bed on the top half shows a little more size variation.

Data and Observations

Initial growth was fairly even for both groups, but as we approached midsummer, it was clear that our multi-plant beds were maturing earlier. this was expected, because of the increased competition they experienced from the other plants within their clusters. When the bulb growth of an onion slows, its top flops to the ground, so this is an obvious signal we can use to gauge the maturity of a crop. At harvest time, I also thought that the multi-plant onions looked a bit smaller, but resisted jumping to any conclusions and waited for the final numbers.

Here's a look at those numbers:

Single Plant Cabernet Onion Bed:

Total dry mass: 19.4kg

Dry mass of bolted onions: 0kg

Number of bulbs: 147

Average mass per bulb: 0.13kg

Multiplant Cabernet Onion Bed:

Total dry mass: 19kg

Dry mass of bolted onions: 0kg

Number of bulbs: 161

Average mass per bulb: 0.12kg

Single Plant Patterson Onion Bed:

Total dry mass: 34.1kg

Dry mass of bolted onions: 2.7kg

Number of bulbs: 160

Average mass per bulb: 0.21kg

Multiplant Patterson Onion Bed:

Total dry mass: 34.9kg

Mass of bolted onions: 0.9kg

Number of bulbs: 180 (Sadly, I have no record of this, but I believe there were close to 180.)

Average mass per bulb: 0.19kg. (That's if there were 180 plants/bed.)


Well, despite my less than perfect record keeping in the end, I am thankful to have done this experiment because there was not as much different between the single and multiplant beds as I expected. The Cabernet single plant onions were 8% larger than their multiplant counterparts and the Patterson single plant onions were 10% larger than their multiplant counterparts. Before you go claiming that single plant onions are always bigger, keep in mind that our plant density wasn't controlled perfectly. The multiplant beds had about 10% more onions, leaving us with about the same total yield per bed. Had we thinned our multiplant beds to contain exactly the same number of onions as our single plant beds, we would have had a more fair comparison. That level of precision will need to wait for another trial.

Now that I have seen these results, I won't have as much hesitation in the future about using the multiplant strategy for onions if we are tight for space in our seed starting nursery. Since there is little change in overall yield with these techniques, it looks like the decision here is mostly a matter of choosing between increased uniformity of the single plant beds or the increased efficiency of the multi-plant beds. I think I'll still go with the single plant beds for the sake of consistency and slightly larger bulbs. What would you choose?

To learn more about how we use soil blocks for seed starting head to our online Classroom. If you have done similar multiplant trials yourself and have numbers to share, I would love to hear from you. Just send me a note with the contact form at the bottom of this page.

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